The MOSDEF Survey: The Nature of Mid-infrared Excess Galaxies and a Comparison of IR and UV Star Formation Tracers at z ∼ 2

Mojegan Azadi,James Aird,Alison Coil,Tom Zick,Mariska Kriek,Bahram Mobasher,Naveen Reddy,Sedona H Price,Gene C K Leung,Ryan L Sanders,Irene Shivaei,William R Freeman,Brian Siana,Alice Shapley

doi:10.3847/1538-4357/aad3c8

Abstract

Abstract We present an analysis using the MOSFIRE Deep Evolution Field (MOSDEF) survey on the nature of “MIR-excess” galaxies, which have star formation rates (SFRs) inferred from mid-infrared (MIR) data that are substantially elevated relative to those estimated from dust-corrected UV data. We use a sample of ∼200 galaxies and active galactic nuclei (AGNs) at 1.40 < z < 2.61 with 24 μm detections (rest-frame 8 μm) from MIPS/Spitzer. We find that the identification of MIR-excess galaxies strongly depends on the methodologies used to estimate IR luminosity ( ) and to correct the UV light for dust attenuation. We find that extrapolations of the SFR from the observed 24 μm flux, using luminosity-dependent templates based on local galaxies, substantially overestimate in z ∼ 2 galaxies. By including Herschel observations and using a stellar-mass-dependent, luminosity-independent , we obtain more reliable estimates of the SFR and a lower fraction of MIR-excess galaxies. Once stellar-mass selection biases are taken into account, we identify ∼24% of our galaxies as MIR excess. However, is not elevated in MIR-excess galaxies compared to MIR-normal galaxies, indicating that the intrinsic fraction of MIR excess may be lower. Using X-ray-, IR-, and optically selected AGNs in MOSDEF, we do not find a higher prevalence for AGNs in MIR-excess galaxies relative to MIR-normal galaxies. A stacking analysis of X-ray-undetected galaxies does not reveal a harder spectrum in MIR-excess galaxies relative to MIR-normal galaxies. Our analysis indicates that AGN activity does not contribute substantially to the MIR excess and instead implies that it is likely due to the enhanced polycyclic aromatic hydrocarbon emission.

Highlights

In the study of galaxies, the star formation rate (SFR) is a fundamental quantity that provides valuable information about the underlying physical processes that drive galaxy evolution.As the energy output from stars is radiated over a wide range of wavelengths from X-ray to radio, the star formation rates (SFRs) of galaxies can be traced at various wavelengths.Ultraviolet (UV) radiation directly traces the population of young, massive stars in a galaxy and can provide a robust tracer of the SFR (e.g., Kennicutt 1998; Calzetti 2001; Magdis et al 2010; Reddy et al 2010)
We further consider whether the SFR traced by Hα is elevated in MIR-excess compared to normal galaxies (Section 3.7)
In this paper we aim to investigate how the templates used for estimating LIR and the choice of the dust correction methods used for estimating SFRUV,corr affect the identification of MIR-excess galaxies

Summary

Introduction

In the study of galaxies, the star formation rate (SFR) is a fundamental quantity that provides valuable information about the underlying physical processes that drive galaxy evolution.As the energy output from stars is radiated over a wide range of wavelengths from X-ray to radio, the SFR of galaxies can be traced at various wavelengths (see, e.g., Kennicutt 1998; Kennicutt et al 2009; Hao et al 2011; Calzetti 2013).Ultraviolet (UV) radiation directly traces the population of young, massive stars in a galaxy and can provide a robust tracer of the SFR (e.g., Kennicutt 1998; Calzetti 2001; Magdis et al 2010; Reddy et al 2010). As the energy output from stars is radiated over a wide range of wavelengths from X-ray to radio, the SFR of galaxies can be traced at various wavelengths (see, e.g., Kennicutt 1998; Kennicutt et al 2009; Hao et al 2011; Calzetti 2013). To reliably estimate the total SFR from the observed UV radiation, one must account for the effects of dust extinction (e.g., Meurer et al 1999; Bouwens et al 2009; Reddy et al 2012, 2015). While less direct than the UV, Hα is less affected by dust extinction and can provide an accurate and reliable SFR tracer (e.g., Glazebrook et al 1999; Kewley et al 2002; Reddy et al 2010; Shivaei et al 2015b)

Objectives

Methods

Results

Discussion

Conclusion